(a) Field of the Invention
The present invention relates to a casting mold including an inorganic water soluble binder and a method for manufacturing the same.
(b) Description of Related Art
It has been commonly known that inorganic water soluble binders such as sulfate compounds are used for forming molds of refractory molding sand particles and the binders are collected for reuse by dissolving the molds in water after the manufacture of castings using the molds. Among the sulfate compounds, an inorganic binder based on magnesium sulfate is used with growing frequency in recent days because the binder can be used repeatedly for the manufacture of aluminum alloy castings and does not generate toxic gases that cause environmental pollution. This tendency is derived from that magnesium sulfate has a melting point of 1185° C. and therefore does not cause decomposition or deterioration of the mold during the manufacture of the aluminum alloy castings.
For example, Japanese Unexamined Patent Publication No. SHO53-119724 describes use of magnesium sulfate heptahydrate as the inorganic water soluble binder. The publication describes that mulled sand made of refractory particles covered with magnesium sulfate heptahydrate is molded and dried at a temperature not lower than 200° C. to obtain a mold. Further, Japanese Unexamined Patent Publication No. HEI11-285777 describes that a mixture of refractory particles, water and a binder made of calcium sulfate and magnesium sulfate is packed in a forming die and heated at 350° C. for 4 hours to obtain a mold.
However, when the above-described magnesium sulfate hydrate is heated to a temperature of 200° C. or higher as described in Japanese Unexamined Patent Publication No. SHO53-119724, the magnesium sulfate loses water of crystallization and turns into magnesium sulfate anhydride. This brings inconvenience in keeping the strength of the resulting mold. According to the inventor's researches, magnesium sulfate in the form of trihydrate or tetrahydrate gives high strength to the resulting mold, but the strength decreases significantly if magnesium sulfate becomes an anhydride. Japanese Unexamined Patent Publication No. HEI11-285777 also describes that a test piece in which magnesium sulfate is used alone as the binder shows a bending strength as low as 0.04 kg/mm2 when heated at 350° C. for 4 hours.
Accordingly, in order to retain the mold strength to a sufficient degree, a huge amount of magnesium sulfate is required. However, this is disadvantageous in shaping and baking of the mold or collection of the binder, resulting in low work efficiency.
Japanese Unexamined Patent Publication No. HEI 11-285777 describes that the combined use of magnesium sulfate and calcium sulfate improves the mold strength. However, calcium sulfate is not suitably used in a water soluble mold because it has low solubility in water.
In contrast to this, the inventor of the present invention has succeeded in baking a mold of high strength such that a certain amount of water of crystallization remains in magnesium sulfate.
In terms of environmental loads and synchronization with casting cycle time, curing in as short time as possible, i.e., fast curing, is required. However, in use of a binder which is based on magnesium sulfate and will be cured by heating, energy for evaporating moisture and raising the temperature of the mold is required, thereby taking long curing time in general. The curing time varies depending on the weight of the intended mold and the molding conditions, but in most cases, the curing time required is about 1 minute for a mold having a mass of about 1 kg, and 5 minutes or longer for a mold having a mass of about 10 kg.
Since magnesium sulfate changes its hydration number depending on temperature, it is necessary to keep every part of the mold at a specified temperature to bake the mold in which magnesium sulfate retains a specified amount of water of crystallization. Accordingly, a longer baking time is required, causing a significant decrease in productivity.
To cope with the above problem, there are known techniques of baking the mold with a microwave or high frequency wave. Such techniques are advantageous in that substances contained in the mold are directly heated to uniform the mold temperature in every part of the mold. However, since the inorganic water soluble binder contains a large amount of moisture and water of crystallization, it takes a long time to raise the mold temperature to 100° C. or higher due to latent heat by moisture vaporization. Moreover, since the heat of the binder is taken by the refractory particles which cannot be heated by the microwave, a temperature difference may be caused within the binder. Therefore, there is difficulty in obtaining a mold of uniform strength.
Further, there are other known techniques of heating a forming die in advance and packing the mulled sand therein to bake the mulled sand with heat transferred from the forming die. According to such techniques, to raise the temperature of the mold making material to a specified temperature higher than 100° C., for example, the forming die is heated to a temperature higher than the specified temperature. However, in this case, the temperature of the surface of the mold making material contacting the forming die exceeds the specified temperature in a short time, but the temperature of the inner part of the mold making material does not rise smoothly, causing a temperature difference between the surface and the inside of the mold making material. Accordingly, if the heating temperature and heating time are controlled such that the surface temperature of the mold making material reaches the specified temperature, the hydration number of the sulfate compound in the inner part of the mold making material remains high. On the other hand, if the heating temperature and heating time are controlled such that the temperature of the inner part of the mold making material reaches the specified temperature, the sulfate compound in the surface part of the mold making material decreases the hydration number or loses the hydration number to become an anhydride. Thus, the hydration number of the obtained mold varies by part and a uniform strength cannot be given to the mold. Therefore, it is necessary to bake the mold making material at low temperatures for a long time to equalize the hydration number in every part of the mold, and thereby giving the uniform strength to the mold.